Hemophilia is a genetic X-linked recessive bleeding disorder caused by a deficiency of a blood clotting factor. There are two basic types of hemophilia; hemophilia A and hemophilia B. Hemophilia A is caused by a deficiency in the blood clotting factor known as Factor VIII and affects approximately 17,000 people in the U.S. Hemophilia B is caused by a deficiency in the blood clotting factor known as Factor IX and affects approximately one out of every 34,500 men. The clinical presentations for both hemophilias are characterized by episodes of spontaneous and prolonged bleeding. Patients frequently suffer joint bleeds which lead to disabling arthropathy. Current treatment is directed at stopping the bleeding episodes with intravenous infusions of plasma-derived clotting factors or, for hemophilia A, recombinant Factor VIII. However, therapy is limited by the availability of clotting factors, their short half-lives in vivo, and the high cost of treatment.
Gene therapy offers the promise of a new method for treating hemophilia. Both types of hemophilia are excellent theoretical candidates for gene therapy as each has a reasonably simple molecular pathology and should be remediable by the delivery of a single gene. Successful gene therapy for hemophilia requires sufficiently high levels of expression of the deficient factor to generate a therapeutic response.
Several groups of researchers have conducted research with gene therapy vectors designed to express Factor IX, but have not been able to achieve stable production of therapeutic levels of these factors in humans. Recently, self-complementary vectors have been used to increase heterologous gene expression by increasing transduction efficiency. See U.S. Patent Pub. No. 2004/0029106, Wang, Z. et al., “Rapid and highly efficient transduction by double-stranded adeno-associated virus vectors in vitro and in vivo”, Gene Therapy 10: 2105-2111 (2003); McCarty, D. M. et al., “Adeno-associated virus terminal repeat (TR) mutant generates self-complementary vectors to overcome the rate-limiting step to transduction in vivo”, Gene Therapy 10: 2112-2118 (2003); McCarty, D. M. et al, “Self-complementary recombinant adeno-associated virus (scAAV) vectors promote efficient transduction independently of DNA synthesis”, Gene Therapy 8: 1248-1254 (2001). However, liver specific expression cassettes currently in use in AAV gene therapy exceed the capacity of these scAAV vectors. See Ziegler, R. J. et al., “AAV2 vector harboring a liver-restricted promoter facilitates sustained expression of therapeutic levels of alpha-galactosidase and the induction of immune tolerance in Fabry mice”, Mol. Therap. 9(2): 231-240 (February 2004); Miao, C. H. et al., “Inclusion of the hepatic locus control region, an intron, and untranslated region increases and stabilizes hepatic factor IX gene expression in vivo but not in vitro”, Mol. Ther. 1(6):522-32 (2000).
Therefore there remains a need for enhancing the production of human Factor IX in vivo using gene therapy vectors to achieve therapeutically effective levels.